The present invention relates to a method of bending a substantially planar, plastically deformable sheet-metal blank or plate to form a shell of cylindrical curvature, and to an apparatus for bending such a blank to a cylindrical curvature.
Long and thick-walled cylindrical vessels, for example steam generators and chemical reactor vessels, are normally manufactured from large, substantially planar sheet-metal blanks. These blanks may have a thickness of from 100-200 mm, a breadth of from 4-5 meters, and a length of 6-12 meters. The blanks are normally shaped in portal presses or four post presses with so-called open tools, i.e. presses in which a punch member is urged down into a die without being brought into contact with a mechanical stop in the final stage of its movement. The die comprises two spaced die bars having a length corresponding to the length of the plate, i.e. 6-12 meters. The punch member is provided with a punch edge having a corresponding length and arranged to penetrate between the die bars. Such an arrangement is shown in FIG. 1.
When shaping the plate, the plate blank is fed stepwise between the punch and the die, and is urged downwardly between the die bars by the punch edge to a predetermined depth determined by the desired shaping radius and the length of the advancing step. The plate can be shaped to a full cylindrical ring having an open slot for the punch, or to a half ring, a so-called half shell. The press forces required for this process are very high, e.g. in order of 5000-20000 tons.
One difficulty encountered in this method of shaping said blanks is that the first part of the blank fed between the punch and the die is not shaped. This also applies to the last part of the plate fed between said die and said punch. The length of these straight parts of the blank can vary, depending upon available press force and plate thickness. A normal value may be 1-3 plate thicknesses. If these straight parts cannot be tolerated in the manufacture of pressure vessels, they must be removed. This is quite usual in normal cases, and results in extra working costs and a large amount of waste.
One method of reducing the length of the straight parts is to use a tool in which the punch edge is displaceable relative to the die bars, so that said punch edge works is closer proximity to the die bar located nearest the end of the plate, as shown in FIG. 2. This known bending method is designated eccentric bending. It gives rise to a number of serious problems, however, namely that the resultant lateral forces are not of equal magnitude, which tends to displace the punch to one side and the die towards the other side. As a result, the press is subjected to much stress and strain, particularly the posts and the guide means.
An object of the present invention is to solve the problem of non-uniform force distribution between the die bars when eccentrically bending a blank in a press of the kind described in the introduction.